1. Field of the Invention
The present invention relates generally to a spray nozzle wherein a plurality of nozzle tip parts cooperate to define a nozzle tip assembly, and to a novel and improved method of manufacturing the nozzle tip parts.
2. Prior Art
In many applications where liquid is to be sprayed, it is important that the liquid be dispersed uniformly and in a precise spray pattern. Uniform dispersion is more readily attained where the liquid being sprayed is highly "atomized", i.e., reduced to a very fine spray. Airless sprayers have been found to be particularly effective in creating a highly atomized spray. In part, this is because airless sprayers utilize high pressures on the order of 1,000 to 3,000 pounds per square inch to force liquid through a very minute nozzle orifice.
Unlike mixing-type spray guns wherein air is mixed with the liquid prior to the liquid's being discharged from the nozzle tip, airless spray guns discharge a homogeneous mixture of liquid through the nozzle tip. Because the liquid is under such great pressure and because the orifice is so small, thorough atomization of the liquid tends to result as the liquid suddenly expands upon discharge from the nozzle orifice. Atomization is sufficiently thorough to provide a spray in the form of a uniform mist of the type desired for use in painting furniture, automobiles, and the like.
Nozzle tips used in airless sprayers are ordinarily formed from a single piece of tungsten carbide steel or a similar hard, wear-resistant material. In order to create precise spray patterns of uniform density, it is important that the orifices in these single-piece tips be formed to exact dimensions. Precision machining a unitary nozzle tip part is an expensive operation which is subject to error and which is highly sensitive to machine tool wear. Inasmuch as nozzle orifices for airless sprayers ordinarily are of quite small dimension and inasmuch as nozzle tips ordinarily are formed from relatively hard materials which are difficult to machine, the formation of single-piece nozzle tips presents a very difficult machining project.
A particular problem exists where it is desired to produce a spray pattern in which a proportionately larger quantity of the liquid is discharged in one direction than is discharged in another direction. For example, it often is desirable to provide an elongate spray pattern of relatively uniform width. Such a spray pattern is useful for spraying large, flat surfaces, for applying chemicals to crops, and for use in various fuel injecting devices. Unfortunately, it has been very difficult to produce a precision, uniform elongate spray pattern with airless sprayers due to difficulties in obtaining perfectly configured nozzle tip orifices.
Another problem with airless sprayers is that of "spitting." This is a condition wherein unacceptably large particles are discharged from the nozzle tip at the start and/or stop of a spraying operation. The tendency of some airless sprayers to spit is so severe that the use of airless sprayers for critical work has been compromised greatly. Indeed, because of this and other spray pattern deficiencies, airless sprayers often are used only for rough work, such as exterior house painting and crop spraying.
It is known to provide a single-piece nozzle tip formed from a blank made utilizing powder metallurgy techniques. The use of powder metallurgy to form blanks is advantageous in several respects. Some metals which will not alloy can be mixed and sintered to provide a hard, wear-resistant material which is well adapted for use in nozzle tips. Moreover, rapid production rates may be employed to provide blanks having a high degree of dimensional accuracy.
Where a single-piece nozzle tip blank is formed through the use of powder metallurgy techniques, a nozzle passage including a precisely configured orifice is machined into the blank by a combination of electric discharge drilling and subsequent diamond milling. The drilling and milling operations are very time consuming and expensive to carry out. Two critical centering operations must be effected during the drilling and milling process, with the result that the reject rate is high. A further deficiency of this present day technique is that the surfaces being machined are inaccessible during machining and rejects are discovered only after all of the costly machining steps are completed. Consequently, the cost of nozzle production is greatly increased due both to a high reject rate and to the fact that valuable machining time is wasted on parts which have become rejects long before the completion of machining.
While some nozzle tip manufacturers form a blank having a rough-formed passage included as part of the blank to save some machining costs, critical and expensive drilling and milling machining operations have still been deemed to be required to finish-form a majority if not all of the nozzle passage, including passage portions on both sides of a nozzle orifice. Moreover, the problem remains of rejects not being discovered until these costly operations are finished.
Perhaps a more fundamental problem encountered with all present day nozzle tip fabrication techniques is that the resultant spray patterns provided by the tips are rarely as precise as desired. With even the finest present day nozzle tips designed to produce an elongate spray pattern of uniform cross-section, the resultant pattern is almost always far less uniform in cross-section than desired. If a painting operation is being carried out, the shape of the spray pattern is such that it is difficult, if not impossible, for the operator of the spraying equipment to overlap sequential spray patterns properly on successive passes to produce a uniform coating of the liquid being sprayed. Consequently, the liquid is applied either too thick or too thin at interface regions between adjacent spray patterns. Many spraying operations have become unduly expensive due to increased operator time required to effect a satisfactory coating of articles being sprayed, excessive material application, and the number of workpieces which must be recoated to overcome coating deficiencies.
One proposal has been made to form nozzle tips from a plurality of nozzle tip parts in order to facilitate machining of the inner surface portions defining a nozzle passage. Such a proposal is presented in British Pat. No. 451,729. In accordance with the proposal described in the British patent, a nozzle tip composed of a number of parts is formed by first machining matingly engageable surfaces on the individual parts. The individual parts are then assembled and a final machining operation is performed to taper the outer surfaces of the nozzle tip parts so that the parts may be held firmly together by a suitable clamping member. Inasmuch as the nozzle tip parts ordinarily are quite small and inasmuch as clamping of these parts together for concurrently machining a taper on their outer surfaces is a difficult operation to perform, the formation of nozzle tips by this technique remains an expensive operation.
An advantage which does obtain through the formation of nozzle tips comprised of a plurality of interfitting parts is that the nozzle passage portions defined by the individual parts can be machined individually as "exterior," readily accessible surfaces using conventional tools. Since nozzle passage portions can be machined "openly" in this fashion, the resulting nozzle passage can be configured quite accurately to provide a desired spray pattern. However, even though nozzle tip parts produced in this manner may provide a greatly improved spray pattern, the technique by which the parts are fabricated results in very high fabrication costs.
A particular problem with the proposal of British Pat. No. 451,729 is that it has been deemed necessary to accurately machine a truncated cone or other tapered surface on the outer walls of the various nozzle tip parts so that, when these parts are clamped together in assembly, a clamping nut or nozzle holder may apply an appropriate and uniform force to the tapered wall portions to tightly clamp the nozzle tip parts together. Prior to the present invention, it has been deemed essential that accurately machined tapered wall portions be provided on outer surfaces of the nozzle parts. Machining the tapered wall portions is an operation which has added significantly to the cost of the resulting nozzle tip.
An approach utilized in conjunction with certain prior art nozzles to improve their spraying characteristics is that of electrostatic spraying. With this technique, an electrostatically charged region is provided forwardly of the nozzle tip at a location as close as possible to the point where atomized liquid particles are discharged from the nozzle tip. Upon passing through the charged region, the particles become charged and tend to travel in a "cloud". This electrostatic charging technique greatly minimizes overspray and tends to facilitate the provision of spray patterns having well defined edges. If a painting operation is being carried out, particles of the paint cloud may be electrostatically attracted to areas which otherwise might not receive paint.
A problem with prior art electrostatic sprayers is that it is difficult to concentrate the electrostatically charged region desirably close to the point where the liquid starts to atomize. In effect, prior electrostatic sprayers have expended much energy in the charging of a field of air surrounding the nozzle tip. Liquid particles passing through the field of charged air are charged essentially indirectly by the air rather than by direct contact with a charging electrode. Previously proposed techniques for charging an atomized spray have not been as effective as desired due mostly to the difficulties encountered in efforts to directly impart a proper charge to the spray.